Such a mixed-flow or “diagonal” fan is known from DE 41 27 134 B4 and corresponding U.S. Pat. No. 5,695,318, HARMSEN, issued 9 Dec. 1997. The fan has a housing that defines, together with the fan wheel of the mixed flow fan, an air flow conduit, within which the fan blades provided on the fan wheel rotate. The fan wheel is also often referred to as an “impeller.”
The enveloping curve of the fan wheel has, for example, a frusto-conical shape, or the shape of a spherical cap. If the drive motor is an external-rotor motor, the hub of the fan wheel is nonrotatably connected to the external rotor of the motor. There remains, between the outer side of the external rotor and the outer side of the fan wheel, an annular cavity, on whose periphery are provided pockets for insertion of balancing weights. It is well known, in the rotating machine art, that rotors wobble the least, and operate most smoothly, when the rotor's center of mass coincides with the central axis of the rotor, and supplemental balancing weights are inserted, when necessary, to adjust for undesired asymmetries which may occur due to manufacturing variations and the like.